1. Field of Invention
This invention relates to a refrigerant type compressor especially for use in automotive air-conditioning systems. More particularly, the present invention relates to a variable displacement mechanism for a slant plate type compressor such as a wobble plate type compressor.
2. Description of the Prior Art
Slant plate type compressors, such as wobble plate type compressors, with variable displacement mechanisms which are suitable for use in an automobile air conditoner are well known in the art. One example is shown in U.S. Pat. No. 3,861,829 issued to Roberts et al. In prior art compressors, the inclination angle of the wobble plate is changed by controlling the pressure in the crank chamber by adjusting the gas pressure on the rear surface of each of the pistons.
Roberts et al. '829 discloses a capacity adjusting mechanism used in a wobble plate type compressor. As is typical in this type of compressor, the wobble plate is disposed at a slant or incline angle relative to the drive axis, nutates but does not rotate, and drivingly couples the pistons to the drive source. This type of capacity adjusting mechanism, using selective fluid communication between the crank chamber and the suction chamber, can be used in any type of compressor which uses a slanted plate or surface in the drive mechanism. For example, U.S. Pat. No. 4,664,604 issued to Terauchi discloses this type of capacity adjusting mechanism in a swash plate type compressor. The swash plate, like the wobble plate, is disposed at a slant angle and drivingly couples the pistons to the drive source. However, while the wobble plate only nutates, the swash plate both nutates and rotates. The term slant plate type compressor will therefore be used to refer to any type of compressor, including wobble and swash plate types, which use a slanted plate or surface in the drive mechanism.
FIG. 1 shows the construction of a conventional wobble plate type compressor. Compressor 1 includes compressor housing 11 having cylinder block 12 at one end. Front end plate 14 is integrally formed with compressor housing 11 to cover the opening at its other end and to form crank chamber 13 within compressor housing 11. Cylinder head 15 is disposed on the opposite end of cylinder block 12 with valve plate 16 disposed therebetween. Drive shaft 2 is rotatably supported by radial bearing 3 in front end plate 14. Central bore 121 is formed in a central portion of cylinder block 12 and the inner terminal end of drive shaft 2 extends within central bore 121. Drive shaft 2 is rotatably supported by radial bearing 4 within central bore 121.
Rotor 5 is fixed on drive shaft 2 within crank chamber 13. Inclined plate 6 is hinged on rotor 5 through hinge mechanism 7 and inclined plate 6 rotates together with rotor 5 and drive shaft 2. The inclination or slant angle of plate 6 is varied by hinge mechanism 7 while it rotates. The slanted surface of inclined plate 6 is in close proximity to the surface of wobble plate 8. Thrust bearing 9 is disposed between the slanted surface of inclined plate 6 and the surface of wobble plate 8 to insure the smooth rotation of inclined plate 6. Guide bar 10 extends within crank chamber 13 from a hole bored in front end plate 14 into a hole bored in cylinder block 12. Lower portion of wobble plate 8 engages with guide bar 10, and wobble plate 8 reciprocates along guide bar 10. Guide bar 10 prevents the rotation of wobble plate 8.
A plurality of pistons 20 are slidably fitted within respective cylinders 17 which are formed through cylinder block 12. Pistons 20 are connected with wobble plate 8 by connecting rods 21. Cylinder head 15 is divided into two interior spaces, suction chamber 151 and discharge chamber 152.
The variable displacement mechanism includes communication passageway 22 which links crank chamber 13 with suction chamber 151, and valve mechanism 23 which is disposed in suction chamber 151 and controls the opening and closing of passageway 22.
As shown in FIG. 2, valve mechanism 23 includes first casing 231 and second casing 232 which is disposed on one open end of first casing 231 and serves to cover the opening. Second casing 232 is provided with communication holes 232a and 232b which provide communication between passageway 22 and suction chamber 151. Bellows 233 is located within the interior space of first casing 231 and is held in position by coiled spring 234. Valve element 235 is fixed to one end surface of bellows 233 and is slidably supported within a hole in supporting plate 236. Valve element 235 controls the opening and closing of cummunication hole 232b of second casing 232.
Supporting plate 236 has a plurality of holes 236a which provide communication between hole 232b and the interior of casing 231. The outer peripheral surface of first casing 231 has at least one aperture 231a which links the interior space of first casing 231 with suction chamber 151. Crank chamber 13 is linked with suction chamber 151 through passageway 22, holes 232a and 232b of second casing 232 of valve mechanism 23, and aperture 231a of first casing 231 of valve mechanism 23 whenever valve element 235 slides to the left position opening communication hole 232b, as shown in FIG. 2.
In the prior art compressor, if the pressure within suction chamber 151 exceeds a predetermined value, bellows 233 within casing 231 contracts, causing valve element 235 to move toward the left. As a result, communication hole 232b is opened allowing crank chamber 13 to be linked with suction chamber 151 through aperture 231a and communication hole 232a. As a result of this link, the pressure in crank chamber 13 is equalized with the pressure in suction chamber 151 which causes the pressure on the rear surface of pistons 20 to be decreased. The decreased pressure on the rear surface of pistons 20 causes the inclination angle of wobble plate 8 to increase allowing the compressor to operate at its maximum capacity.
If the pressure in suction chamber 151 falls below the predetermined value, bellows 233 within first casing 231 expands and extends toward the right in FIG. 2. As a result, valve element 235 closes communication hole 232b and the communication link between crank chamber 13 and suction chamber 151 is terminated. The pressure in crank chamber 13 gradually increases, and the pressure on the rear surface of pistons 20 is increased, and as a result the inclination angle of wobble plate 8 is decreased. The reduced inclination angle of wobble plate 8 causes the compressor to operate at a reduced capacity.
When the prior art compressor is used in an automobile air-conditioning apparatus, if there is a high thermal load in the passenger compartment when compressor operation begins and the engine is driven at a high revolution rate, the pressure in suction chamber 151 rapidly decreases below the predetermined value even if the passenger compartment has been insufficiently cooled. Specifically, the variable displacement mechanism will operate to terminate the link between suction chamber 151 and crank chamber 13 even if the temperature in the passenger compartment is greater than desired.
As shown in FIG. 3, the cooling characteristics of the prior art compressor with a variable displacement mechanism are inferior to those of the conventional compressor without a variable displacement mechanism. Both the temperature in the passenger compartment and in the air outlet louver is significantly lower for the conventional compressor without the variable displacement mechanism. In addition, the pressure in crank chamber 13 is drastically changed in order to change the inclination angle of wobble plate 8. It is possible that the lubricating oil contained within crank chamber 13 will flow into suction chamber 151, and undesirable result.